Update README.md (#9)

* Update README.md

Add more description to "promote" argument of deploy-appengine action

* Update README.md

Accepting suggesting to remove double wording

Co-authored-by: Averi Kitsch <[email protected]>

Co-authored-by: Averi Kitsch <[email protected]>

Author: oiegorov-geeksforless

README.md

@@ -59,7 +59,8 @@ steps:
   by this deployment. If you do not specify a version, one will be generated for
   you.
 
-- `promote`: (Optional) Promote the deployed version to receive all traffic.
+- `promote`: (Optional) Promote the deployed version to receive all traffic. 
+  Possible values: ''|'true'|true|'false'|false, if not specified behavior defaults to promote.
 
 ### app.yaml customizations